The study was originally designed to look at the genetic and behavioral toxicity of methylphenidate hydrochloride, the key ingredient in ADHD medications.

Scientists working on the study noticed the rhesus monkeys given a high dose of MPH showed significantly lower testosterone levels, significantly slower rates of testicular decent and reduced testes size.

The study’s authors point out that male rhesus monkeys and male humans have similar puberty processes and are therefore a good model for how the drug may affect humans.

The effects of the drug were not permanent however, and the authors insist the findings are preliminary, according to MSNBC.

“This was an unexpected finding,” said study researcher Dr. Donald Mattison, of the Eunice Kennedy Shriver National Institute of Child Health and Human Development. The original premise of the study was to examine whether methylphenidate damaged DNA, Mattison said.

The findings are very preliminary, and should be interpreted with caution, Mattison said. Additional studies should attempt to replicate the results.

Parents of children who take this drug for ADHD should talk with their child’s doctor before making any changes to their son or daughter’s medication, Mattison said.

About 3 to 5 percent of school-aged children have the condition which is characterized by a lack of attention, hyperactivity and impulsive behavior. More boys are affected than girls. The most effective treatment for ADHD combines medication with behavioral therapy, according to the National Center for Biotechnology Information.